Anorectal Fistula

Anal fistula occurs most commonly when the anal glands, which reside in the intersphincteric plane, become occluded and infected, resulting in a cryptoglandular abscess. Whether surgically or spontaneously drained, a perirectal abscess may still result in a fistula in up to 40% of cases; however, spontaneously draining abscesses tend to have a higher fistula rate, up to 66%.[2] The mean incidence has been reported at 8.6 per 100,000. The presence of an acute or chronic anal fistula can be distressing for patients and cause reduced quality of life. They are commonly classified by anatomical location, as first described by Parks, Gordon, and Hardcastle in 1976. Understanding anorectal anatomy and the classification of perirectal fistulas is paramount for their management. Anorectal fistulas are classified into transphincteric, high intersphincteric, suprasphincteric, and extrasphincteric types, based on the location of the tract in relation to the internal and external anal sphincters as described by Parks and Gordon. Most abscesses form in the space between the internal and external sphincters, so intersphincteric fistulas end up being the most common. In this pattern, the tract passes through the internal sphincter before continuing outward to an external opening on the skin. A fistulotomy efficiently manages these (laying open of the fistulous tract) and rarely causes incontinence, as the treatment does not affect the external sphincter. Intersphincteric fistulas are the most common type of fistula, comprising 50-80% of all cryptoglandular fistulas. Trans is a Latin word for “on the other side of.” So a trans-sphincteric fistula crosses to the other side of the external sphincter before exiting in the perianal area and thus involves both sphincters. Transsphincteric fistulas pose a challenge in management and often require more complex or staged treatment. However, the use of a seton to gradually “lower” the tract and reduce its involvement with the external sphincter may allow migration of the tract and a fistulotomy at a later date, while preserving the patient's continence. The extent of involvement of the external sphincter dictates the likelihood of postoperative incontinence, as a partial sphincterotomy is usually tolerated. Still, if the fistula involves the majority of the sphincter, then incontinence results after a complete division. These fistula tracts travel superior to the external sphincter and cross the puborectal muscle before changing course caudal to their external opening. Accordingly, they pass the internal sphincter and the puborectal muscle but spare the external sphincter. When these patients typically present with a perirectal abscess, it may not be visible on inspection, but they have tenderness on the digital rectal exam. Again, because of their high tract, a seton may be considered before fistulotomy in these cases. A fistulectomy is similar to fistulotomy but involves the removal of the entire fistula tract either sharply or with cautery. Historically, radical fistulectomy was the standard treatment for anal fistula; however, fistulotomy tended to be preferable as it preserved more sphincter function, was a less morbid procedure, and healed faster. However, more recently, with specialists performing the majority of fistula procedures, the outcomes of fistulectomy and fistulotomy appear similar. A recent meta-analysis published in 2016 of 6 randomized controlled trials comparing fistulectomy versus fistulotomy in low fistulas found no significant difference in recurrence in 5 trials and no significant difference in postoperative incontinence in 4 trials. Extrasphincteric fistulas typically originate in the more proximal rectum rather than the anal canal and often develop as sequelae of prior procedures. The external opening appears in the perianal region, while the tract ascends to enter the anal canal above the dentate line. The St. James University Hospital (SJUH) classification is an imaging-based system that grades fistulas according to anatomy. MRI plays a central role here: its superior soft-tissue contrast allows clear visualization of the sphincter complex and the fat planes of the perirectal and supra-levator spaces. MRI also facilitates more accurate identification of the internal opening because of its ability to image in multiple planes. Published in 2000 by Morris et al, the SJUH system mirrors the Parks classification in some areas while offering more detailed stratification based on MRI findings. Grade 1 represents a simple linear intersphincteric fistula, identical to the classic “intersphincteric” pattern in the Parks system. When a grade 1 fistula is associated with an abscess or a secondary tract, it is categorized as grade 2. Crossing of the external sphincter defines a grade 3, or trans-sphincteric, fistula. If a trans-sphincteric tract is accompanied by an abscess or secondary extension into the ischiorectal fossa, it is considered grade 4. A grade 5 fistula extends above or through the levator ani musculature, forming supra-levator or trans-levator pathways. Importantly, MRI-based classification has been shown to predict surgical outcomes more reliably than intraoperative findings alone. Management generally aligns with the degree of sphincter involvement. Grade 1 and 2 fistulas are usually amenable to fistulotomy or fistulectomy; if an abscess is present, the abscess is incised and drained first. Persistent fistulas after resolution of acute infection are then treated according to their type. Grades 3 and 4 involve the external sphincter and may require seton placement followed by fistulotomy or a sphincter-sparing procedure such as LIFT or an endoanal advancement flap. Grade 5 fistulas often reflect atypical underlying pathology and warrant further evaluation before operative intervention. Fistulas related to Crohn disease require management tailored to the underlying inflammatory disorder and are addressed separately.